Power transformers, and other large transformers, are a key component of the power transmission system in an electric utility. Several thousand such transformers may be in use at a major facility. A single power transformer may represent an investment of millions of dollars, so a failure can be extremely costly in outage time and investment loss. Transformers are designed to last about 40 years. The residual life of an operating transformer is typically dependent on the residual life of the solid insulation in the transformer.
The solid insulation for the coils in most power transformers and other large transformers is oil impregnated paper. This paper is immersed in a dielectric fluid, such as transformer oil. By monitoring the condition of the paper insulation component it is possible to schedule and minimize maintenance procedures and adjust the power level input in order to extend the useful life of the transformer and reduce maintenance costs.
As the insulation paper ages by thermal degradation (typically, transformers are rated to operate at temperatures between 55.degree. C. and 65.degree. C. above ambient), the cellulose in the paper breaks down. Specific degradation compounds from this breakdown process appear and build up in the transformer oil. The most prominent is a build up of furaldehyde (also known as furfuraldehyde, furfural, fural or 2-furaldehyde) and related furaldehyde such as 5-methyl 2 furaldehyde and 5 hydroxy-2 furaldehyde. By monitoring the amount of furaldehyde build up in the oil, the condition (or proportional degree of degradation) of the paper insulation can be assessed.
The presence of furaldehyde build-up in the insulating oil of transformers as an indicator of paper insulation degradation, has been known for some years. Around 0.1 to 0.5 parts per million of furaldehyde in the oil is the range that is now considered as an indication that cellulose in the paper insulation is degrading at a significant rate. The importance of being able to detect furaldehyde at such low levels is now becoming appreciated by maintenance engineers.
In some power utilities equipped with laboratories, transformer oil samples are routinely monitored for the presence of furaldehyde as an indicator of paper degradation. The conventional procedure employs high performance liquid chromatography (HPLC), which provides comprehensive information on numerous paper degradation products and is sensitive to very low levels of furaldehyde (as low as 0.01 ppm). But the HPLC procedure is slow and expensive and requires a trained technician operator.
The detection of furaldehyde in new, clean petroleum products, which do not have any significant oxidation breakdown products, has been accomplished in the petroleum industry by using an aniline in acetic acid reagent and measuring the colour produced with a photometer, or alternatively by visual examination of the colour in the acetic acid layer which separates out underneath the petroleum product layer. A problem with the acetic acid/aniline test is that the mixture solidifies at about 16.degree. C. Therefore, the test is not useful outdoors in cold climates.
Transformer oil, unlike the clear thin petroleum products that are usually tested with the acetic acid/aniline test, for example, gasoline, kerosene and the like, presents a challenge because transformer oil is viscous which retards reaction rates. Also, transformer oil oxidizes and darkens under use. The transformer oil in a typical transformer may be 5 to 40 years old. Since they operate at elevated temperatures, for example, up to 105.degree. or higher at localized spots, the build-up of oxidation breakdown products can be significant. These breakdown products can form emulsions which interfere with the test. In the case of viscous transformer oil, which retards movement of the acetic acid reagent, a diluent solvent can be used to reduce the viscosity of the oil so that the acetic acid reagent can more effectively contact the furaldehyde in the oil.
To increase the sensitivity of the aniline-acetic acid test, the furaldehyde can be pre-concentrated by extracting a large volume of the oil with a small volume of a suitable solvent. This extracted material is then used for conducting the analysis. This additional step increases the complexity of the procedure and requires a trained technician operator.
Using the alternative photometric colour measurement procedure described is relatively tedious, time consuming and requires a photometer and a trained technician operator. The test is advantageous, however, because when the procedure is used to detect furaldehyde in oxidized transformer oil, a minimum test sensitivity as low as about 0.05 ppm of furaldehyde can be achieved.
Visual colour measurement, if it could be done reliably, is desirable because it is simple, much less tedious, and does not require any instruments or a trained technician operator. But, because it is optical, it is inherently less sensitive. When used to detect furaldehyde in transformer oils, the sensitivity is further reduced because: (a) the use of a diluent solvent reduces the furaldehyde concentration, making detection more difficult, (b) yellowish or brownish coloured oxidation products from the oil are extracted by the acetic acid and these mask the true intensity of the furaldehyde red/pink compound and reduce the visual detection limit of the test, and (c) the oxidation products form emulsions which obscure the distinction between the upper oil layer and the lower pink/red coloured layer.
To date, the minimum test sensitivity of visual colour detection of furaldehyde in transformer oil, without resorting to an additional pre-concentration step, has only been about 0.5 to 1.0 ppm of furaldehyde depending on the colour condition (oxidation level) of the oil. The visual test is therefore not sufficiently sensitive to enable a meaningful paper degradation assessment to be carried out, particularly under cold field conditions. As mentioned before, a serious problem in using an aniline/acetic acid test is that when aniline and acetic acid are mixed, the mixture freezes at +16.degree. C. This limits the applicability of the test in indoor or outdoor conditions when temperatures are below +16.degree. C.
As the significance and usefulness of furaldehyde detection, as evidence of cellulose breakdown, to monitor the condition of paper insulation in transformers and other electrical equipment, has become more appreciated, and hence more important, the demand for analysis has increased substantially. This has led to the need for a simple reliable, accurate test that can be conducted visually in the field by non-chemistry trained personnel to rapidly detect the presence of low levels of furaldehyde in transformer oil that has been in use in transformers for years.
There is, to the applicant's knowledge, no quick and reliable visual field test for detecting low levels of furaldehyde in oxidized viscous transformer oil.
U.S. Pat. No. 4,514,503, issued Apr. 30, 1985, R. B. Orelup, discloses a two-component liquid reagent comprising a first component and a second component for detecting the presence of furfural in new, clear light petroleum products of low viscosity, such as gasoline, kerosene, diesel oil, and the like. Orelup uses diethylene glycol in each component to lower the freezing point of the mixture. The freezing points of the two components are stated to be less than -40.degree. C. Each component of the liquid reagent is stored separately from the other component and both components are combined with each other prior to admixture with the petroleum product. The test is intended for use by tax authorities to monitor unauthorized blending of motor fuels with less expensive products such as low octane gas and heating fuels, the latter having a lesser tax rate.
The two components of Orelup comprise the following compositions on a weight basis:
First Component: (a) from about 15 to about 22 volume percent of a primary amine selected from the group consisting of aniline, meta-aminophenol, para-anisidine, meta-toluidine and para-toluidine; (b) from about 35 to 45 volume percent of diethylene glycol; (c) from about 35 to about 45 volume percent of ethanol; and (d) from about 1 to 2 weight to volume percent of an antioxidant. PA1 Second Component: (a) from about 18 to about 25 weight to volume percent of an acid selected from the group consisting of citric acid, lactic acid, formic acid and phosphoric acid; (b) from about 35 to about 45 volume percent of diethylene glycol; and (c) from about 35 to 45 volume percent of ethanol. PA1 1. The three reagents are stored separately to improve stability and shelf life. The colour that is developed in an upper layer when the reagents are used in detecting furaldehyde is stable, does not fade or change, and is less susceptible to contamination from a layer containing petroleum products, than is a lower indicator layer. PA1 2. The test sensitivity is sufficiently great that detection of furaldehyde even in diluted and aged mineral oil can be made by visual means, without pre-concentration steps, in concentrations as low as about 0.1 parts per million of furaldehyde. A comparison of detection limit in viscous transformer oil, which must be diluted, is the most relevant comparison because in diesel and heating oil, a lower furaldehyde detection level is easier to achieve because a diluent solvent is not required, and the diesel or heating oil is not viscous and is a new clean product. PA1 3. Interference from coloured oil oxidation products in the degraded oil and formed emulsions is substantially reduced because the colour indicator appears in an upper layer. PA1 4. The analytical procedure for low level semi-quantitative measurement of furaldehyde in mineral oil is simplified and is much more rapid than prior art tests. PA1 5. The kit and analytical procedure can be used in both indoor and outdoor conditions, even at extremely low temperatures (for example, -40.degree. C.). PA1 6. The reagent components are held in sealed airtight containers which ensure reagent stability, ease of use and operator safety. PA1 7. The volume of transformer oil and reagents required for the test is minimal and therefore a compact, portable multiple sample test kit package is possible. PA1 (a) First Component PA1 (b) Second Component PA1 (c) Third Component
There is a proviso in Orelup that when the amine in the first component is aniline, then the acid in the second component must be an organic acid selected from the group consisting of citric acid, lactic acid and formic acid. This is probably to avoid the high freezing point (+16.degree. C.) of the aniline/acetic acid mixture. The use of diethylene glycol (which is a well known and widely used anti-freeze in sprinkler systems and automotive radiator systems) and ethanol in each component is also taught. It is claimed that this method with no pre-extraction can detect 0.25 ppm of furaldehyde in new clean gasoline, diesel fuel, kerosene, naptha, or heating oil, all of light viscosity.
These light petroleum products are clear in colour and are fresh. They are not old or degraded in any way. They are not heavy hydrocarbons of high viscosity.
The test taught by Orelup produces a petroleum product upper layer and a separated lower layer which displays a red colour if there is a furaldehyde primary amine reaction. The lower indicator layer is prone to interference from emulsions formed by degradation products.
Orelup discloses the presence of inhibiting and diluting diethylene glycol in both the first component and the second component. Orelup also discloses large amounts of diluting ethanol in both the first component and the second component. The presence of additional chemicals in each component dilutes the concentrations and reduces the sensitivity and the reliability of the procedure.